1. Field of the Invention
The present invention relates to a position detection apparatus, a position detection method, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto a substrate such as a wafer by a projection optical system.
To keep up with demands for advances in micropatterning of semiconductor devices, the projection exposure apparatus is being required to improve the resolving power (a minimum feature size that it can transfer). Along with this trend, the wavelength of the exposure light is shortening, high-NA projection optical systems are under development (the numerical aperture (NA) of the projection optical system is increasing), and the exposure region is widening.
Along with advances in micropatterning of semiconductor devices, the projection exposure apparatus is also being required to align the reticle and the wafer with an accuracy as high as ⅓ the line width of the circuit pattern. For example, a typical current circuit designed to have a line width of 90 nm must be aligned with an accuracy of 30 nm.
The exposure apparatus performs the alignment by transferring an alignment mark onto a wafer, together with the circuit pattern, and detects the position of the alignment mark in transferring the next circuit pattern onto the wafer, thereby aligning the wafer with the reticle. The alignment mark transferred onto the wafer is optically detected by, for example, a position detection apparatus (image sensing apparatus) 1000 as shown in FIG. 21. FIG. 21 is a schematic view showing a conventional position detection apparatus 1000.
In the position detection apparatus 1000, light emitted by a light source 1002 is deflected by a beam splitter 1004, and illuminates an alignment mark (target object) 1010, which is transferred onto a wafer 1008, via an imaging optical system 1006. The light reflected by the alignment mark 1010 forms an image on an image sensor 1012 via the imaging optical system 1006, and is sensed by the image sensor 1012 as the optical image (light intensity distribution) of the alignment mark 1010.
A known technique corrects deterioration in the optical image (light intensity distribution) of the alignment mark 1010 attributed to the optical system of the position detection apparatus 1000, and restores an optical image of the alignment mark 1010 by measuring the optical transfer function of the imaging optical system 1006 in advance and using an inverse filter. For example, letting f(x) be the optical image (light intensity distribution) of the light reflected by the alignment mark 1010, g(x) be the optical image (light intensity distribution) on the image sensor 1012, and H(ω) be the optical transfer function of the imaging optical system 1006, we have:G(ω)=H(ω)×F(ω)  (1)where G(ω) is the Fourier transform of g(x), F(ω) is the Fourier transform of f(x), and H(ω) is the Fourier transform of a point image distribution function (PSF: Point Spread Function) or a line image distribution function (LSF: Line Spread Function) and represents the imaging characteristic of the imaging optical system 1006.
Equation (1) is rewritten as:F(ω)=G(ω)×1/H(ω)  (2)
Inverse Fourier transformation of equation (2) yields an optical image before it deteriorates due to the optical system of the position detection apparatus 1000, that is, the optical image f(x) of the light reflected by the alignment mark 1010.
To increase the accuracy of the position detection apparatus, Japanese Patent Laid-Open No. 2004-281904 proposes a technique of measuring, for example, the optical transfer function and electrical transfer function of the position detection apparatus in advance, thereby correcting deterioration in the optical image (light intensity distribution) using these two transfer functions.
Unfortunately, in the prior art, if the reflectance of a target object such as an alignment mark has a wavelength dependence, it is impossible to satisfactorily correct deterioration in the optical image (light intensity distribution) attributed to the optical system of the position detection apparatus, resulting in a decrease in the detection accuracy of the target object.
For example, first, the prior art measures, using broadband light as the illumination light, the PSF or LSF over the entire wavelength width of the illumination light. Next, deterioration in the optical image is corrected using an optical transfer function G(ω) as the Fourier transform of the PSF or LSF. Note that the optical transfer function G(ω) which includes the information of the influence of aberrations in the optical system has a wavelength dependence. This is because aberrations (for example, an on-axis chromatic aberration) in the optical system generally differ in their generation amounts (aberration amounts) among wavelengths. For this reason, if differences in light intensity occur among wavelengths between when the PSF or LSF is measured and when a target object such as an alignment mark is measured, it is impossible to satisfactorily correct deterioration in the optical image (light intensity distribution).
In addition, as the wafer has a stacked structure in which transparent thin films such as a resist and interlayer dielectric film are stacked, the reflectance with respect to each wavelength varies, as shown in FIG. 22. FIG. 22 is a graph showing the wavelength dependence of the reflectance of the wafer (alignment mark) serving as the target object. In FIG. 22, the ordinate indicates the reflectance of the wafer, and the abscissa indicates the wavelength of light which illuminates the wafer. To reduce the influence of multiple reflection by these transparent thin films, the position detection apparatus illuminates the wafer with illumination light having a wavelength width, and detects the position of the alignment mark transferred onto the wafer. However, Japanese Patent Laid-Open No. 2004-281904 does not take account of the wavelength dependence of the reflectance of the wafer (alignment mark). Therefore, even when the PSF or LSF is calculated over the entire wavelength width of the illumination light, and an optical transfer function G(ω) as its Fourier transform is used, the optical image (light intensity distribution) changes depending on the wavelength. This makes it impossible to satisfactorily correct deterioration in the optical image.